The present invention relates to an exposure apparatus and more specifically relates to a device for holding an optical element in an exposure apparatus.
Manufacturing methods of microdevices such as semiconductor devices, liquid crystal display elements, image pickup devices or thin film magnetic heads, and manufacturing methods of masks such as reticles or photomasks include a photolithography process using an exposure system. In the photolithography process, substrates (wafers or glass plates) on which a photosensitive material or a photoresist is applied, are treated. More specifically, in the photolithography process, a pattern on a reticle is illuminated by an illumination optical system. Then the image of the pattern is transferred to each of shot regions defined in the substrate through a projection optical system.
Recently, the manufacture of highly integrated semiconductor devices requires the transfer of a fine pattern. Thus, the exposure apparatus requires a projection optical system which has extremely small wave front aberration and distortion.
To satisfy the requirements, a prior art exposure apparatus 100 shown in FIG. 23 includes a drive mechanism 95 that adjusts the position of an optical element 92. That is, the exposure apparatus 100 includes a barrel 91, a plurality of lenses 92 (92a, 92b) accommodated in the barrel 91 and an optical element holding device 93 that holds two lenses 92a placed near a reticle R. By the optical element holding device 93, the two lenses 92a are moved in the optical axial direction, which is shown by the broken line, and their optical axes are inclined with respect to the optical axial direction. Further, other lenses 92b, which are placed in the intermediate portion of the barrel 91 and in the vicinity of a wafer W, are fixed to the barrel body 91a. 
FIG. 25 shows a holding mechanism of the lenses 92a. The lenses 92a are accommodated in a sub-barrel 91b. The sub-barrel 91b is connected to the top end of the barrel body 91a and is movable in the axial direction through three plate springs 94. One end of each plate spring 94 is bolted to the barrel body 91a (or a static sub-barrel 91c of FIG. 24) by a bolt 98, and the other end of the plate spring 94 is bolted to the sub-barrel 91b by a bolt 98. A plurality of actuators 95 (FIG. 23) including piezo-electric elements are arranged around the barrel body 91a. The actuators 95 cause the lenses 92a to move in the optical axial direction together with the sub-barrel 91b. On the outer surface of the barrel body 91a are arranged a plurality of sensors 96 in the vicinity of the actuators, respectively. The sensors 96 detect the position and attitude of the sub-barrel 91b. 
The lens 92a is moved by the actuator 95 in the optical axial direction with the sub-barrel 91b. This enables the efficient manufacture of the projection optical system including the barrel 91. Further, in the exposure apparatus equipped with such a projection optical system, changes in various aberrations or distortions due to changes in the atmospheric pressure and illumination heat occur. However, it is possible to correct the changes in the aberrations and distortions easily in real time in the exposure process.
However, in the exposure apparatus 100, only the lens 92a that is placed in the vicinity of the reticle R is moved by the actuator 95. Therefore, the types of optical aberrations to be corrected are limited.
The group of lenses 92b, which is arranged in the intermediate portion of the projection optical system, has a larger influence on the image forming properties when moved in the optical axial direction or the tilt direction than the group of lenses 92a, which is arranged at the top portion of the projection optical system and in the vicinity of the reticle R. In other words, the movement of the lenses 92b in the optical axial direction or the tilt direction greatly affects the image formation property. Therefore, if the lens group 92b in the intermediate portion of the projection optical system is driven, a higher drive performance and guide precision than in the lens group 92a near the reticle R are required for its drive control. In the exposure apparatus 100 it is difficult to meet such requirements.
To move the lens 92b in the intermediate portion of the projection optical system, additional sub-barrel, which accommodates the lens 92a and 92b, is required. The sub-barrel 91b is movably supported on the additional sub-barrel. Thus, even if in the barrel 91, the sub-barrel 91b can be driven, it was difficult to move the additional sub-barrel with the sub-barrel 91b. 
The inventor of the present invention proposed an exposure apparatus 200 shown in FIG. 24, which includes a projection optical system having three lenses 92a and three lenses 92b. By driving a plurality of stacked movable sub-barrels 91b, the lenses 92a accommodated in the movable sub-barrels 91b can be moved. However, in the exposure apparatus 200, a static sub-barrel 91c accommodating the lenses 92b cannot be stacked directly on the movable sub-barrel 91b. As a result, each static sub-barrel 91c is supported on the lower static sub-barrel 91c through a supporting member 97.
However, in the exposure apparatus 200 shown in FIG. 24, a supporting member 97 having a larger diameter than the barrel 91 increases the size of the barrel 91. Further, since the actuator 95 and sensors 96 corresponding to the movable sub-barrels 91b are arranged in the supporting member 97, the maintenance, exchange and inspection of the actuator 95 and sensors 96 are intricate.
Additionally, when the specific wave front aberration or distortion component is corrected, driving at minimum five portions is required in principle and five optical element holding devices 93 having movable sub-barrels 91b are needed. In this case, it becomes difficult to ensure the accommodation space for lenses 92b in the barrel body 91a. 
Further, when the actuator 95 is a piezo-electric element with high precision, low heat generation, high stiffness and high cleanliness, the actuator 95 is relatively long and parallel to the optical axis. This may necessitate enlargement of the barrel body 91a. Accordingly, as the actuator 95, a voice coil motor, or a fluid pressure drive, which is compact and has a large movable range, may be used.
However, since the voice coil motor generates heat in operation, the lens 92a is not positioned precisely and various aberrations due to the heat are simultaneously generated. On the other hand, when the fluid pressure actuator is used, the stiffness for supporting the movable sub-barrel 91b may be insufficient. Thus, the vibration of the outer portions of the exposure apparatus may affect the movable sub-barrel 91b so that the control responsibility for the movable sub-barrel 91b is degraded. In particular, in a recent scan type exposure apparatus, a reticle stage and a wafer stage are driven at high speed. As a result, a relatively large load acceleration acts on the barrel. Thus, maintaining stiffness in supporting the movable sub-barrel 91b is indispensable.
As described above, in the exposure apparatuses 100 and 200 use three plate springs 94 as a guide mechanism for the optical element holding device 93. In a construction shown in FIG. 26, slippage between the respective ends of the plate spring 94 and the corresponding barrels 91a to 91c cannot be avoided. Thus, when the actuator 95 allows the lens 92a to move in the optical axial direction together with the sub-barrel 91b, the plate spring 94 is bent. In this case, it is actually impossible to restrict slippage on the contact surfaces between the respective ends of the plate spring 94 and the corresponding barrels 91a to 91c to a sub μm order with only the fastening force of the bolts 98.
The main factor of this slippage is a cosine error due to bending of the plate spring 94, as shown in FIG. 27. When the plate spring 94 of length L, one end of which is fixed, is bent by an angle of only α, the horizontal direction distance between both ends of the bent plate spring 94 is shorter than the plate spring 94 before it is bent (when it is straight), by only the cosine error L(1−cos α). Thus, a sub μm order slippage which compensates for the cosine error L(1−cos α), occurs between the plate spring 94 and the barrels 91a to 91c. 
When the lens 92a and the sub-barrel 91b are moved in the optical axial direction, they are also displaced by the mounting conditions, materials and variation in size of the three plate springs 94, in the radial direction as shown in FIG. 28.
If the amount of displacement in the radial direction is extremely slight, its influence on the image formation performance is small. However, when the amount of displacement in the radial direction exceeds a predetermined value, an image shift is generated on the wafer W so that registration precision is degraded. In this case, after measuring the amount of displacement of the lens 92a and the sub-barrel 91b previously, the displacement is corrected with a wafer stage so that the registration precision can be ensured. However, in order to make this correction possible, the correspondence between one part displacement with respect to the optical axial direction to one part displacement with respect to the radial direction, that is, the facts that reproducibility can be obtained in the displacement with respect to the radial direction to the displacement with respect to the optical axial direction and there is no hysteresis is a condition. This reason is that the correction of the image shift with the wafer stage is not a closed loop control which measures the positions of the image in real time, but an open loop control which monitors the displacement with respect to the optical axial direction.
However, since slippage occurs between the plate spring 94 and the barrels 91a to 91c, there is hysteresis in the displacements of the lens 92a and sub-barrel 91b, as shown in FIG. 28. Thus, it is difficult to correct the image shift with the wafer stage and to ensure the registration precision.
Further, from an optical viewpoint the tolerable degree of displacement is generally relatively large near the reticle R, but small in the intermediate portion of the projection optical system. Therefore, driving the lens group in the intermediate portion of the projection optical system requires a significantly higher precision than in driving the lens group near the reticle R. As a result, it is difficult to correct aberrations with high precision with the exposure apparatus shown in FIG. 24.